The present invention relates to a magnetic shield room and, more particularly, to a magnetic shield room having an opening portion through which wafers and the like are unloaded and loaded.
Conventionally, as a unit for forming a resist pattern on a sample such as a wafer or mask, an EB (Electron Beam) exposure unit which draws a pattern on a photoresist by using an electron beam is used.
An EB exposure unit of this type has a problem that the electron beam irradiation position varies by the influence of the external magnetic field to undesirably cause a distortion in the drawn pattern. In order to prevent this, such an EB exposure unit is arranged in a shield room to operate while it is shielded from the external magnetic field.
Concerning a wafer or mask to be processed by the EB exposure unit, a cassette which stores a plurality of wafers or masks is usually handled as one unit. Therefore, the EB exposure unit has a loading portion for extracting a wafer or mask as a processing target from the cassette and supplying it to the processing chamber, and an unloading portion for storing a processed wafer or mask in the cassette. Also, a means for attaching and detaching the cassette to and from the loading and unloading portions is necessary.
When such loading and unloading portions are arranged in the magnetic shield room, conventionally, the cassette is manually loaded and unloaded by the operator or the like through an inlet/outlet port formed in the magnetic shield room and provided with a normally closed door.
When the door is opened to allow the operator to enter or leave the room, the EB exposure unit is influenced by the external magnetic field. Therefore, when loading/unloading the cassette, operation of the EB exposure unit must be temporarily stopped, leading to a decrease in throughput.
FIG. 6 shows a conventional magnetic shield room. As shown in FIG. 6, a maintenance door 3 is provided to the side wall of a magnetic shield room 1, and a magnetic field shield material is adhered to the inner wall of the magnetic shield room 1. An EB exposure unit (not shown) or the like is arranged in the magnetic shield room 1.
With this arrangement, to prevent the influence of the external magnetic field, an opening portion having such a size that it does not allow the external magnetic field to influence the EB exposure unit may be formed in the magnetic shield room 1, and the cassette may be loaded/unloaded through this opening portion.
Based on the demands for a higher micropatterning degree and a higher integration degree in recent semiconductor integrated circuits, a strict pattern drawing precision of 0.1 .mu.m or less has been required, and the magnetic field around the EB exposure unit must be suppressed as low as possible.
If an opening portion having a size required for loading/unloading a cassette (e.g., one having a size of 180 mm.times.180 mm.times.180 mm) is formed, the external magnetic shield enters the magnetic shield room 1 through the opening portion to adversely affect the EB exposure unit. Therefore, the EB exposure unit must be installed to be sufficiently remote from the opening. As a result, the area occupied by the magnetic shield room 1 with respect to the area occupied by the EB exposure unit becomes considerably large.
In order to set the strength of the entering external magnetic field not to influence the EB exposure unit, the length of the short sides of the rectangular opening portion must be decreased to 100 mm or less. With this size, however, at most only one wafer or mask can be passed through this opening portion, and wafers and masks stored in a cassette cannot be loaded/unloaded at all.
As a method of suppressing entrance of the external magnetic field into the magnetic shield room 1 through the opening, one in which a tubular magnetic field shield material is provided to the outside of the opening portion is proposed, as shown in Japanese Patent Laid-Open No. 59-197198.
In order to improve the magnetic shield effect without decreasing the opening ratio, a technique as shown in Japanese Utility Model Laid-Open No. 3-12497 is proposed, in which a stereoscopic shield lattice having a depth is further arranged in a tubular shield material arranged outside the opening portion, such that the interval pitch is smaller than the depth.
In order to sufficiently shield the external magnetic shield by providing a tube made of a magnetic shield material at the opening portion, the length of the tubular member must be increased in accordance with the size of the opening. If, however, the tubular member is long, it interferes with the operability of loading/unloading the cassette in/from the loading and unloading portions in the magnetic shield room. For example, when the operability of placing the cassette on the loading or unloading portion is considered, the length of the tubular member is preferably as small as possible.
When a shield lattice is arranged in the tubular member, it is suitable for an application such as a vent port. For an application, e.g., a case that includes loading/unloading of a cassette, the lattice interval must be increased. For this reason, the length of the tubular member must be increased in accordance with the size of the opening, thus interfering with the operability.